The invention relates to a cathode ray tube having an envelope comprising a phosphor screen on one side and a neck portion on the other side, and an electron gun positioned in the neck portion and having a beam-forming part and a focusing structure, said beam-forming part comprising at least a cathode and a metal electrode plate provided with a central aperture, said focusing structure comprising a hollow tube of an electrically insulating material with inner and outer surfaces and with a layer of resistive material on at least one of the surfaces.
A cathode ray tube of this type is known from EP-A 233,379, corresponding to U.S. Pat. No. 4,857,797. The cathode ray tube described in this Specification has an electron gun comprising a hollow glass tube. During manufacture the glass tube is softened by heating it and is drawn on an accurately made mandril whose diameter changes several times in the longitudinal direction. Abutment faces for the electrodes of the beam-forming part of the gun are formed on the inner side of the tube thus calibrated. The focusing structure is formed by a resistive layer which is provided in a helical shape on the inner wall of the glass tube.
If such a "glass" gun is made in large quantities, the very accurately made (and hence costly) mandrils required during manufacture appear to be subject to rapid wear. This is at the expense of reproducibility. Moreover, it appears to be a problem to construct the electrode components to be inserted in the glass tube with a sufficiently constant shape.